1. Field of the Invention
The present invention relates to a light emitting diode having a heterogeneous material structure and a method of manufacturing thereof, and more specifically, to a light emitting diode having a heterogeneous material structure and a method of manufacturing thereof, in which efficiency of extracting light to outside is improved by forming depressions and prominences configured of heterogeneous materials different from each other before or in the middle of forming a semiconductor material on a substrate in order to improve the light extraction efficiency.
2. Background of the Related Art
Recently, lighting instruments or the like configured with light emitting diodes (LED) have a long lifespan, consume low power compared with conventional incandescent or fluorescent lamps, do not discharge contaminants in the manufacturing process, and thus demands thereon explosively increase. The light emitting diodes are applied to backlight elements of lighting apparatuses or LCD display apparatuses, as well as display apparatuses which use luminescence, and their application fields are gradually diversified.
The light emitting diode is a kind of solid element which transforms electrical energy into light and generally includes an active layer of a semiconductor material intervened between two opposing doping layers. If bias is applied to the two doping layers, holes and electrons are injected into the active layer, and light is emitted when the holes and the electrons are recombined. The light generated at the active layer is emitted in all directions out of a semiconductor chip through all exposed surfaces.
FIG. 1 is an exemplary view showing a process of manufacturing a general light emitting diode. As shown in FIG. 1a, after growing an n-type semiconductor layer 102 on a sapphire substrate 101, an active layer of an InGaN/GaN nitride multiple quantum well 103 structure is formed to emit light by combination of electrons and holes, and a p-type semiconductor layer 104 is formed thereon.
Next, as shown in FIG. 1b, mesa etching is performed using photo lithography in order to form an electrode by exposing an area of the n-type semiconductor layer 102.
Then, as shown in FIG. 1c, an n-type electrode 105 is formed on the top of the n-type semiconductor layer 102 of the mesa etched area, and a thin p-type metal film 106 capable of transmitting light is coated on the p-type semiconductor layer 104. Then, a thick p-type electrode 107 is deposited on the top of the p-type metal film 106.
As an important means for enhancing light extraction efficiency in such a conventional structure, particularly in a nitride-based LED structure, approaches to maximize internal quantum efficiency of the active layer and approaches to extract light generated in the active layer out of the LED chip as much as possible are tried.
There are largely two factors which determine the light extraction efficiency in a light emitting diode. The first one is optical loss in a current diffusion layer related to a transmittance degree, and the second one is optical loss caused by total reflection on an interface through which light is emitted.
In relation to the first factor, an Ni/Au alloy layer or the like having a thickness of some nm to some tens of nm is frequently used as a current diffusion layer in a conventional LED, and it has a transmittance of 60 to 80% with respect to an emission wavelength depending on thickness and alloy conditions.
Although an approach to use an electrode material of high transmittance such as ITO or the like is made recently in order to overcome such a phenomenon, there is a problem in commercializing the electrode material due to high contact resistance against the p-type semiconductor (GaN) layer, and thus techniques employing a structure such as an n-p tunnel junction or an InGaN/GaN super lattice are applied.
The optical loss caused by total reflection in relation to the second factor is generated due to difference in refractive index between adjacent materials at an interface which emits light from the top of a light emitting diode to outside, i.e., an interface between p-type GaN and resin, p-type GaN and air or other material contacted with the p-type GaN, or an interface between a buffer layer and the sapphire substrate positioned at a lower part of the LED element.
A semiconductor constituting a general light emitting diode has a high refractive index compared with external environments such as a substrate, an epoxy, an air layer and the like, and thus most of photons generated by combination of electrons and holes stay inside the (LED) element. Therefore, external quantum efficiency is greatly affected by the structural shape of the element and optical characteristics of the materials constituting the element.
Particularly, since the refractive index of a material constituting a nitride semiconductor light emitting diode is higher than that of the materials surrounding the light emitting diode (e.g., air, resin, substrate, etc), the photons generated inside the light emitting diode do not escape to outside and is absorbed inside the light emitting diode, and thus the light emitting diode has low external quantum efficiency (extraction efficiency).
One of conventional approaches for reducing optical loss generated by total reflection is a method of patterning a shape of regular or irregular depressions and prominences in an area of a p-type GaN layer using a process such as etching or the like in order to change an incident or emission angle at an interface.
However, particularly, in the case of a nitride semiconductor optical device, when the surface is coarsely processed in a dry or wet etching method due to the relatively thin p-type semiconductor layer, a defect can be induced in the active layer placed right below the p-type semiconductor layer, and contact resistance can be increased due to changes in the characteristics of the p-type semiconductor layer.
Therefore, in the case of a nitride semiconductor, since there is a limit in the method of coarsely processing the surface after growing a thin film, a method of coarsely processing the surface in the course of depositing a thin film is employed.
This is a method which improves brightness compared with a conventional LED by evenly growing the substrate and the layers formed thereon except the last thin film and densely forming pits on the surface by changing growth conditions such as a III-V group ratio, temperature, deposition speed and the like when the last thin film is grown.
However, this method has a problem in that the process of densely forming the pits is complicated.